1. Field of the Invention
The present invention relates to a reflective optical system, and more particularly, to a reflective optical system which is suited to image an object on the ground or in the sky onto an imaging plane by using light with a wide wavelength range from ultraviolet to infrared and which has a F-number of 2 to 4 and a field angle of coverage of approximately 3.degree. to 4.degree..
2. Description of the Related Art
Reflective optical systems have been frequently used as a high-performance imaging apparatus for imaging a target (object) on the ground or in the sky. In such systems, a Schmidt camera can easily attain relatively high optical performance.
The Schmidt camera is provided with an optical system in which the center of curvature of a concave spherical mirror (main mirror) functions as a diaphragm position and an aspherical correction plate for correcting spherical aberration is disposed near the center of curvature. This structure substantially eliminates off-axial coma and astigmatism and offers relatively high optical performance for a ray having a wide field angle.
There are problems in the Schmidt camera that an image plane in an effective ray bundle curves in a spherical shape, and that chromatic aberration is caused by the aspherical correction plate. Particularly, when the Schmidt camera is used in a wide wavelength range from ultraviolet to infrared, more chromatic aberration owing to the aspherical correction plate arises. Therefore, it is required that the whole optical system be constituted by only reflecting mirrors.
A reflective optical system using a reflecting correction plate instead of the refractive correction plate of the Schmidt camera is suggested in, for example, U.S. Pat. No. 4,576,452.
FIG. 6 is a schematic view of the optical system in a Schmidt camera suggested in the above patent which uses an aspherical reflecting correction plate. Referring to the figure, a ray bundle from an object is reflected by an aspherical correction plate 61, and reflected and concentrated by a spherical mirror (main mirror) 62, thereby forming an image of the object on an imaging plane 63.
In addition, Applied Optics, No. 17,141 and others suggest a reflective optical system which folds a ray bundle from a main mirror by a convex mirror in the opposite direction to let an imaging plane out of an effective ray bundle.
The former reflective optical system suggested in U.S. Pat. No. 4,576,452 has a problem that the aspherical correction plate made of an off-axis mirror is hard to produce since the surface thereof is rotationally asymmetrical.
On the other hand, in the latter reflective optical system suggested in Applied Optics, the effective aperture of the convex mirror is large, and a substantial F-number is large since the effective ray bundle is blocked by the convex mirror.